This invention relates to an improved construction for use in a plastic mold type of semiconductor device.
Generally, such semiconductor device as IC, LSI, etc. is manufactured by the steps of dividing a single semiconductor wafer on which a plurality of semiconductor elements are formed into separate chips (dicing step), mounting respective chips on respective packages or lead frames (die bonding step), interconnecting an electrode pad on each chip and an lead on each package or a lead frame with a fine gold wire or a fine aluminum wire (wire bonding step) and finally sealing with plastic resin respective chips and a portion of a lead connected thereto (sealing step).
In the die bonding step, which is performed prior to the wire bonding step, chips are secured to a ceramic package or to a die pad of a lead frame. By this operation, mechanical and electrical connections between the chips and the package can be made readily, and heat dissipation of their parts can be improved. Generally the die bonding step is desired that the connected parts (junctions) should have a high electroconductivity and a low thermal resistance and that the junction is physically and chemically stable. To form a die bonding satisfying these conditions the following three methods have been used, namely gold-silicon eutectic alloy method, soldering method and resin bonding method.
Since the gold-silicon eutectic alloy has a relatively low melting point of 370.degree. C., it is widely used in the die bonding. The bonding is effected by pressing the rear surface of the chip to a gold plated die pad while heating them at a temperature of about 400.degree. C. in an atmosphere of N.sub.2 or a mixture of N.sub.2 +H.sub.2 which are used for the purpose of preventing oxidation so as to cause gold-silicon eutectic reaction necessary for bonding. Since with this method a high degree of mechanical and electrical connections between the chip and the package can be obtained, this method is used widely. The soldering method utilizes a Pb-Sn type solder instead of the gold-Si eutectic alloy, and its operating temperature is relatively low, for example 200.degree.-300.degree. C. According to this method, a film of a Ni-Au alloy or a Ti-Ni-Au alloy which are wettable with solder, is formed on the rear surface of a chip by a metallizing technique, a small piece of a Pb-Sn type solder is interposed between the chip and a base metal layer, for example a plated silver layer, on a die pad, and the assembly is heated in an atmosphere comprising a mixture of N.sub.2 and H.sub.2 for bonding together the chip and the die pad. This method has the advantage that the thermal strain caused by the difference in the thermal expansion coefficients of a substrate and silicon is absorbed by the interposed solder layer so that even when the chip area of LSI, for example, becomes large, breakage of the chip would not occur.
According to the third method, a bonding agent (adhesive) is used. Generally, epoxy resin incorporated with a silver powder is used. The bonding agent is required to have such characteristics that it does not generate any gas giving an adverse influence upon the elements at the time of hardening and that it has a high heat conductivity and an electric conductivity. To ensure better bonding, the die pad is plated with gold or silver which decreases electric contact resistance. Since the setting temperature of the epoxy resin is about 150.degree.-200.degree. C., the bonding operation can be made at a lower temperature than the first and second methods so that it is possible to mount a large chip capable of absorbing heat strain caused by the difference in the thermal expansion coefficients. For this reason, the third method is widely used for automating the operation and reducing the cost of assembling.
In any one of the three methods, gold or silver is generally plated on the lead frame.
FIG. 2 is a plan view showing a typical prior art plated leads frame 10 which is constituted by a plurality of leads 5, a die pad 2 for mounting chips and tie bars 3 for supporting the die pad 2. An assembly of these circuit elements constitutes a unit, and a plurality of units are combined in series to form an entire lead frame. This lead frame is sealed with plastic resin within the region designated by a reference numeral 6. Lead portion inside of the plastic mold is called as inner lead and that outside of the plastic mold is called as outer lead. A region to be plated is shown by a hatched portion 4, and this region includes the entire portion of the die pad 2, tie bars 3 adapted to interconnect the lead frame 1 and the die pad 2 and inner portions (corresponding to the bonding portions) confronting the die pad 2. Among the materials utilized for the lead frame 1 are included Cu and 42 alloy which is a ferrous alloy containing 42% by weight of Ni. As above described Au, Ag, Ni, etc. are employed as plating materials. Furthermore, since the inner leads are connected to bonding wires for increasing the bonding forces between them, the inner leads are plated with Ag, for example. This measure is necessary for any one of the three methods described above.
In this manner, in the step of die bonding for mounting a chip (semiconductor element) on the bed frame of the lead frame, it is a general practice to plate the entire die pad with metal so that forming a satisfactory plating is an essential factor of this step.
According to a conventional method, since the entire surface of the die pad is plated, the area of the plated pattern or region is wide resulting in a longer plating time and increase in the cost of the lead frame. Moreover, when a plurality of lead frames plated with silver are stacked, lead frames touching each other would be bonded together thus forming a defective product. At the present time, tie bars are also plated with metal, for example silver. Where a plated pattern is formed a misalignment would occur between the lead frame and the plating pattern. In such a case, the plated portion often protrudes to the outside of the molded resin. Then silver migration would occur at such portion, thus often short circuiting the other tie bars.